1. Field of the Invention
The invention relates to a method and circuit for evaluating the output signals of subscriber line current flow detectors in a telephone exchange.
2. Description of the Related Art
The detection of current flows is a criterion which is often required in electrical circuits. The flow of subscriber line loop direct current and/or ringing alternating current can be detected in the telephone exchange by means of current flow detectors. West German Pat. No. DE-AS 23 59 867 discloses a circuit arrangement in a telephone exchange for detecting direct current on which an alternating current is superimposed, in which the flow of the loop direct current and/or of the ringing alternating current can be detected by means of opto-couplers and the output signals of the current flow detectors are evaluated by an evaluation circuit. The zero-crossings of the loop current determined by the current flow detectors are used in the evaluation circuit for the purpose of supervision.
In addition, West German Pat. No. DE-AS 29 25 409 describes a circuit arrangement for detecting answering by a called subscriber, in which such answering can also be detected during the transmission of the ringing current. The subscriber set is remotely supplied from the switching exchange via the subscriber connecting lines. The amplitude of the loop direct current then depends, as does also the amplitude of the ringing alternating current, on the length of the subscriber lines. What is commonly referred to as loop current recognizers (current flow detectors) are connected to a subscriber line, it then being possible to check on the basis of the logic value of the potential of the detector output signals whether loop direct current flows. For the detection of ringing alternating current, opto-couplers are provided which are connected to a subscriber line via the contact of a relay of the ringing current generator and isolating capacitors. For the detection of loop direct current and ringing alternating current the output signals of the loop current recognizer and of the opto-coupler are applied to an evaluation circuit, constituted by an extensive gate logic circuit. Checking whether a ringing alternating current is superposed on the loop direct current is effected during the zero-crossing of the ringing alternating current. The opto-couplers, which are connected to anti-parallel to each other, are arranged in the direct current-free branch of the ringing current supply line in parallel with a resistor provided in the ringing current supply line.
The circuit arrangement disclosed in the aforesaid West German Pat. No. DE-AS 29 25 409 has the disadvantage that the circuit cost and design efforts for the evaluation circuit are comparatively high and that the output signals produced by the loop current detectors are only considered to be valid when the opto-couplers detect a zero-crossing of the ringing alternating current. Noise induced in the subscriber connecting line, for example common-mode voltages, may produce faulty results in the evaluation circuit.
In addition, the circuit arrangement described in the aforesaid West German Pat. Nos. DE-AS 23 59 867 and DE-AS 29 25 409 have the disadvantage that hidden interferences produced by line interruptions or a short-circuit to earth in the subscriber connecting line are not detected in a flowing ringing alternating current.